Antifuse with electrostatic assist

ABSTRACT

A structure and method for providing an antifuse which is closed by laser energy with an electrostatic assist. Two or more metal segments are formed over a semiconductor structure with an air gap or a porous dielectric between the metal segments. Pulsed laser energy is applied to one or more of the metal segments while a voltage potential is applied between the metal segments to create an electrostatic field. The pulsed laser energy softens the metal segment, and the electrostatic field causes the metal segments to move into contact with each other. The electrostatic field reduces the amount of laser energy which must be applied to the semiconductor structure to close the antifuse.

This application is a divisional of U.S. patent application Ser. No.09/702,406, filed on Oct. 31, 2000, which issued as U.S. Pat. No.6,498,056.

TECHNICAL FIELD

This invention relates generally to integrated circuits and morespecifically to an apparatus and method for providing an antifuse.

BACKGROUND OF THE INVENTION

In integrated circuit manufacturing, it is often desirable to selectparticular circuits from an array. For example, a pair of redundantparallel circuits may be formed and, after testing one circuit to verifythat it functions properly, the other parallel circuit may be removed.One method currently used to remove the unwanted circuit is to formprogrammable fuse elements (i.e., sacrificial metal lines buried in adielectric layer) in the circuits which are normally closed, then toblow a fuse element by vaporizing the fuse element with laser energy toopen the circuit that is not selected. This technique can cause rapidheating around the fuse, however, shattering the fuse cover and creatingan explosive release of metal particles. This explosive release of metalparticles can leave residue on the chip surface, creating the risk ofcontamination problems in subsequent process steps.

An alternative method is to form a normally open path, then to close thepath to form an antifuse if the circuit is selected. One antifusestructure is disclosed in U.S. Pat. No. 5,793,095 issued to Harvey.Disclosed is the use of laser energy to fuse together two or moreconductive terminals, through an optically transparent dielectric layer,to close a circuit (i.e., to form an antifuse). The disclosed antifusestructure requires that the conductive terminals to be fused must belocated in the same metal layer. Therefore, it can not be used to forminterlayer connections. In addition, the structure requires either agreat deal of laser energy, which can cause damage, or that theterminals be very close, which can cause leakage current and capacitivecoupling.

Another approach that incorporates antifuse structures is to apply laseror incoherent energy to damage a dielectric layer separating twoconductive terminals. This approach uses a different closure mechanismthan the present invention. In U.S. Pat. No. 5,811,869 issued to Seyyedyet al. and U.S. Pat. No. 5,528,072 issued to Boudou et al., laser energyis used to damage a dielectric layer to create a short between an upperconductor and a well or doped region of a substrate. Because thesestructure can only be formed in a substrate, they are not useful forinterlevel connections and all levels above the substrate must be freeof wiring above the antifuse. To create a short circuit, the terminalsmust be very close, which could cause leakage current and capacitivecoupling absent the antifuse.

U.S. Pat. No. 5,270,251 issued to Cohen shows a method for providing aprogrammable link having a breakdown voltage (and consequently aprogramming voltage) that can be reduced by applying incoherentradiation to a composite insulator between metal layers in the link.Cohen teaches that specific limitations must be applied to the insulatorformed between a bottom conductor and a top conductor, such as materialand structural limitations. Cohen does not teach or suggest either ahorizontal structure or laser energy. Also, Cohen forms ahigh-resistance link, which can adversely effect circuit performance.

U.S. Pat. Nos. 5,314,840 and 5,485,032, each issued to Schepis et al.and assigned to the assignee of the subject invention, teach a methodand structure for forming a programmable antifuse comprising adjacentbodies of germanium and aluminum or aluminum alloy. The germanium isheated using resistance heating or irradiation, causing it to alloy withthe aluminum and form a low-resistance connection. This method requiresthe use of germanium and aluminum. Also, the high energy input requiredto alloy the germanium and the aluminum may be detrimental to somecircuits which require antifuses.

U.S. Pat. No. 4,617,723 issued to Mukai discloses a method for forming aconductive link comprising a metal layer and an undoped polysiliconinsulating layer. The polysilicon is caused to react with the metal toform a conductive silicide link by applying heat with a laser beam. Thisprocess requires complex multi-poly processes and high activationtemperatures.

The deficiencies of the conventional antifuse structures and of theconventional methods incorporating such structures show that a needstill exists for improvement. To overcome the shortcomings of theconventional structures and methods, a new antifuse structure and a newmethod for closing or switching an antifuse structure are needed.

It is an object of the present invention to provide a low-resistanceantifuse structure and method which do not cause contamination insubsequent processes. It is a further object of the present invention toprovide a low-resistance antifuse structure and method with low-energylaser pulses and electrostatic assist which minimize energy input intothe chip containing the antifuse. It is yet another object of thepresent invention to provide a low-resistance antifuse structure andmethod which are compatible with low dielectric constant (low-K)interlayer dielectrics.

SUMMARY OF THE INVENTION

To achieve these and other objects, and in view of its purposes, thepresent invention provides an antifuse structure and method usinglow-energy laser pulses and electrostatic assist to close the antifuse.In one embodiment of the present invention, two metal segments areformed having edges in close proximity. The metal segments can beembedded in material which is a poor diffusion barrier to the metalsegments, such as polyimide or porous silicates for unclad copper.Alternatively, an air gap can be formed around the metal segments usinga sacrificial fill technique. One or more of the metal segments isexposed to low-energy laser pulses which soften the metal segments, anda voltage potential is applied between the metal segments, causing themetal to flow and bridge the gap between the metal segments.

In another embodiment of the present invention, a first metal segmentand a second metal segment are formed within an antifuse cavity filledwith an interlayer dielectric, and the interlayer dielectric is removedusing a process such as a wet etch. The second metal segment is exposedto low-energy laser pulses which soften the metal segment, while avoltage potential is applied between the first and second metalsegments, causing the second metal segment to bend and to contact thefirst metal segment.

The present invention provides considerable improvement over the priorart. One advantage is that the present invention can provide alow-resistance antifuse which is closed with a minimum energy input intothe chip containing the antifuse. Another advantage of the presentinvention is that it is compatible with new low-K dielectrics. Also, thepresent invention does not cause contamination in subsequent processingsteps.

It should be understood that both the foregoing general description andthe following detailed description are exemplary, but are notrestrictive, of the invention.

BRIEF DESCRIPTION OF THE DRAWING

The features and advantages of an antifuse structure and method forcreating an antifuse with electrostatic assist will be more clearlyunderstood from the following description when read in connection withthe accompanying drawing. Included in the drawing are the followingfigures:

FIGS. 1 through 3 show sequential top views of a structure and methodfor providing an antifuse with electrostatic assist according to oneembodiment of the present invention, in which

FIG. 1 illustrates two metal segments formed over a semiconductorstructure with a fuse gap between the metal segments,

FIG. 2 illustrates a voltage potential applied between the metalsegments, and

FIG. 3 illustrates how the electrostatic force between the metalsegments, created by the applied voltage potential, causes the metal inthe metal segments to flow when subjected to laser heating and bridgethe fuse gap, creating a permanent conductive link;

FIGS. 4A through 8B show sequential views of a structure and method forproviding an antifuse with electrostatic assist according to anotherembodiment of the present invention, in which

FIG. 4A illustrates a first metal layer, having a first metal segmentwith a lower terminal extension, formed over a semiconductor structure,

FIG. 4B is a sectional view taken along the line 4B—4B of FIG. 4A,

FIG. 5A illustrates an interlayer dielectric, in which cavity sidewallsand a second metal segment are formed, placed over the first metalsegment,

FIG. 5B is a sectional view taken along the line 5B—5B of FIG. 5A,

FIG. 6A illustrates an antifuse cover formed over the interlayerdielectric, the cavity sidewalls, and the second metal segment andpatterned to form an upper contact opening, a laser window opening, andan etchant opening,

FIG. 6B is a sectional view taken along the line 6B—6B of FIG. 6A,

FIG. 7A illustrates an antifuse cavity, formed by removing theinterlayer dielectric through the etchant opening by etching, and aninterlayer dielectric, deposited over the structure, in which an uppercontact is formed contacting the second metal segment,

FIG. 7B is a sectional view taken along the line 7B—7B of FIG. 7A,

FIG. 7C is a sectional view illustrating an optional embodiment with theantifuse cavity extending above the second metal segment,

FIG. 8A illustrates a voltage potential applied between the first metalsegment, through the lower terminal extension, and the second metalsegment, through the upper contact during application of pulsed laserenergy to the second metal segment through the laser window opening, and

FIG. 8B is a sectional view taken along the line 8B—8B of FIG. 8A;

FIG. 9 illustrates a top view of a horizontal embodiment of the presentinvention before closing the antifuse with the antifuse cover omittedfor clarity;

FIG. 10 illustrates a top view of a horizontal embodiment of the presentinvention after closing the antifuse with the antifuse cover omitted forclarity;

FIG. 11 is a top view of a multi-state antifuse structure according toyet another embodiment of the present invention with the antifuse coveromitted for clarity; and

FIG. 12 is a cross-sectional view of the antifuse structure of FIG. 11taken along line 12—12.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be described in detail with reference to theaccompanying drawing in which like reference numerals designate similaror corresponding elements, regions, and portions. The present inventionprovides an antifuse structure and a method for switching the antifuseusing an electrostatic assist, reducing the laser power required forswitching.

First Embodiment

Referring now to FIG. 1, two metal segments (21, 31) are formed over asemiconductor structure (11) with a fuse gap (40) between metal segments(21, 31). Semiconductor structure (11) should be understood to possiblyinclude a wafer or substrate of semiconducting material such asmonocrystalline silicon or a like structure known in the art, such assilicon-on-insulator (SOI). Semiconductor structure (11) should beunderstood to possibly further include one or more conductive orinsulating layers over a substrate or the like, as well as one or moreactive or passive components on or over a substrate or the like. Metalsegments (21, 31) can be any of a number of conductive metals used inthe art for interconnections, including but not limited to copper andaluminum. The metal segments (21, 31) can be formed by a blanketdeposition and patterning or by a damascene process, as is known in theart.

The fuse gap (40) can be an air gap, formed by depositing a sacrificialfill layer such as a polymer place holder and removing the sacrificialfill layer with, for example, an oxygen (O₂) reactive ion etch (RIE).Alternatively, fuse gap (40) can be filled with a dielectric material,which is a poor diffusion barrier to the metal in the metal segments(21, 31), such as polyimide or a porous silicate for unclad copperwiring. The width of the fuse gap (40) is preferably between about 0.25microns and 0.50 microns, although a larger gap can be used with ahigher voltage potential. Also, a smaller gap can be used provided thesacrificial fill layer can be removed and the current leakage remainswithin acceptable limits.

Referring now to FIG. 2, a voltage potential (45) is applied betweenmetal segments (21, 31), and one or both metal segments (21, 31) areheated with laser pulses to soften metal segments (21, 31). Voltagepotential (45) must be sufficient to form a field which will cause massmotion. The voltage required for such a field varies inversely with themagnitude of laser energy applied to metal segments (21, 31) (i.e., thegreater the laser energy applied to metal segments (21, 31), the lowerthe potential that is required to form a field which will cause massmotion and close the antifuse). For example, a potential of 1 voltacross a gap of 0.25 microns will create an electric field of about 4MV/m, which is sufficient to close the antifuse when a laser energydensity of between about 0.1 Joule/cm² and 1.0 Joule/cm² is applied tometal segments (21, 31). This energy density can be provided by a laserpower of between about 10⁵ Watts/cm² and 10⁶ Watts/cm² for pulses ofbetween about 0.1 microseconds and 1 microsecond. This relatively lowpower laser, as compared to conventional laser-closed antifuses, avoidsthermal gradients in metal segments (21, 31) that can cause stress anddamage.

Because laser energy is dissipated quickly in metal wiring, single pulseenergy is preferred. The pulse length of the laser energy can beadjusted between about 0.1 microseconds and 1 microsecond to provide theoverall energy required. For aluminum-containing metal segments (21,31), an ArF excimer laser is preferred, providing laser energy at awavelength of about 193 nanometers because aluminum absorbs energy wellat 193 nanometers. Other lasers can be used, however, with an adjustmentto the energy recipe.

Voltage potential (45) creates an electrostatic force between metalsegments (21, 31). The force causes the metal in metal segments (21,31), softened by laser energy, to flow and bridge fuse gap (40). Thus, apermanent conductive link (50) is created. Permanent conductive link(50) is shown in FIG. 3.

Optionally, one or both of metal segments (21, 31) can be formed with anirregular shape having one or more corners along the edge facing fusegap (40). The irregular shape will enhance the electromagnetic field dueto the potential gradient around each corner. Another option is to applyheat to the entire semiconductor structure, during antifuse closure, toreduce the amount of laser energy required. For example, thesemiconductor structure can be heated to between about 100° C. and 200°C. for short periods of time to reduce the laser energy required withoutdamaging most semiconductor structures.

Second Embodiment—Vertical Antifuse

A second embodiment of the present invention is shown in FIGS. 4Athrough 8B. Referring to FIGS. 4A and 4B, a first metal layer (220) isformed over a semiconductor structure (11). First metal layer (220) cancomprise any metal used for interconnect wiring in semiconductors,including but not limited to copper and aluminum. First metal layer(220) is patterned to form a first metal segment (221) with a lowerterminal extension (222) extending from first metal segment (221).

Referring to FIGS. 5A and 5B, an interlayer dielectric (240) is formedover first metal segment (221). Interlayer dielectric (240) preferablycomprises silicon oxide as is known in the art, but may comprise anorganic dielectric such as polyimide, polyarylene ether, or otherdielectric materials as are known in the art. Next, interlayerdielectric (240) is patterned to form cavity sidewall openings thatcontact first metal segment (221) and extend upward to the top ofinterlayer dielectric (240). The cavity sidewall openings are filledwith a metal plug (i.e., a via) to form cavity sidewalls (251).Sidewalls (251) of the antifuse structure are continued upward bypatterning the next wiring level(s) to form the metal sections.Optionally, the bottom of the cavity can be an etch-resistant dielectricin which case lower terminal extension (222) is connected elsewhere onthe cavity sidewall.

A second metal segment (252) is formed in interlayer dielectric (240)within, but not contacting, the cavity sidewalls (251) or first metalsegment (221). Second metal segment (252), which is preferably formedusing a damascene process, is circumscribed by interlayer dielectric(240) and cavity sidewalls (251). Second metal segment (252) preferablyhas a length of less than about 10 microns, and is approximatelyequidistant from sidewalls (251) along its length and about 1 micronfrom first metal segment (221). Although these geometries are compatiblewith the voltage potential and the laser energy recipe supplied in thefirst embodiment, it should be understood that other geometries andrecipes are within the scope of the present invention.

Referring to FIGS. 6A and 6B, an antifuse cover (260) is formed overinterlayer dielectric (240), cavity sidewalls (251), and second metalsegment (252). Antifuse cover (260) preferably comprises silicon nitrideformed in two layers. Antifuse cover (260) is patterned to form an uppercontact opening (261), a laser window opening (263), and an etchantopening (265). Interlayer dielectric (240) within cavity sidewalls (251)is removed through etchant opening (265) using a wet etch selective tointerlayer dielectric (240) over the material of cavity sidewalls (251),second metal segment (252), and antifuse cover (260). Such removal formsan antifuse cavity (270). When dielectric layer (240) comprises anorganic dielectric, removal may be accomplished using anoxygen-containing plasma or reactive ion etch.

Referring to FIGS. 7A and 7B, an interlayer dielectric (280) isdeposited over the semiconductor structure. Etchant opening (265) has anarrow neck to minimize the flow of a subsequently formed secondinterlayer dielectric into antifuse cavity (270), sealing antifusecavity (270) under interlayer dielectric deposition pressure. A methodfor forming etchant opening (265) is provided in “Vacuum-sealed SiliconMicromachines Incandescent Light Source,” C. H. Masrangelo and R. S.Muller, IEDM proceedings, p. 503 (1989), which is incorporated byreference. Interlayer dielectric (280) can be formed such that laserenergy can be transmitted through it as disclosed in the publication byMasrangelo and Muller, foregoing the need for an explicit laser windowopening. In this way, antifuse cavity (270) is hermetically sealedduring fabrication, and does not require more processing after the fuseblow. An upper contact (282) is formed in interlayer dielectric (280)contacting second metal segment (252).

Referring to FIGS. 8A and 8B, a voltage potential (285) is appliedbetween first metal segment (221), through lower terminal extension(222), and second metal segment (252), through upper contact (282).Voltage potential (285) forms an electrostatic field between first metalsegment (221) and second metal segment (252). With voltage potential(285) applied, pulsed laser energy (287) is applied to second metalsegment (252) through laser window opening (263) to soften second metalsegment (252). The electrostatic field causes softened second metalsegment (252) to move into contact with first metal segment (221),closing the antifuse. The voltage required varies inversely with themagnitude of laser energy applied to second metal segment (221). Thepreferred voltage potential and laser energy recipe is approximately 1Volt and between about 0.1 Joule/cm² and 1.0 Joule/cm², although itshould be understood that other voltage potential and laser energyrecipes can be derived that are within the scope of the presentinvention.

FIG. 8B shows the end of second metal segment (252) moving into contactwith first metal segment (221). It should be understood that, due toadhesion of second metal segment (252) to antifuse cover (260), themiddle of second metal segment (252), rather than the end, could moveinto contact with first metal segment (221). The length of second metalsegment (252) must be sufficient to allow contact to first metal segment(221) if adhesion occurs.

Referring to FIG. 7C, an optional second dielectric layer (not shown)may be formed over second metal segment (252) and cavity sidewalls (251)can be extended upward through the second dielectric layer beforeforming antifuse cover (260). Both the dielectric layer and the seconddielectric layer are removed using an etch, resulting in antifuse cavity(270) extending above second metal segment (252). A key advantage ofantifuse cavity (270) extending above second metal segment (252) is areduced risk that second metal segment (252) will adhere to antifusecover (260). Adhesion could adversely effect antifuse closure.

Second Embodiment—Horizontal Antifuse

As shown in FIGS. 9 and 10, the antifuse can alternatively be formedhorizontally, so that first metal segment (221) and second metal segment(252) are formed from the same metal layer. In the horizontal antifuse,first metal segment (221) is formed in at least one of the sidewallopenings. Second metal segment (252) may be formed much closer to onesidewall than to the other if the cavity sidewalls are to beelectrically connected. Also, in the horizontal antifuse, the bottom ofthe cavity may comprise a dielectric, such as silicon nitride, ratherthan a first metal layer.

Referring now to FIG. 9, dielectric layer (240) overlying thesemiconductor structure (not shown) is patterned to form sidewallopenings and a second metal segment opening. The sidewall openings havea first terminal extension opening. The sidewall openings, firstterminal extension opening, and second metal segment opening are filledwith a metal, such as by deposition and CMP planarization to form firstmetal segment (221) which is also the sidewalls of the antifuse cavity,a first terminal extension (222), and a second metal segment (252).Optionally, the cavity sidewalls are extended above and below secondmetal segment (252) using additional metal segments (e.g., via andwiring metal).

Referring now to FIG. 10, a antifuse cover is formed over the antifusecavity and patterned to form an upper contact opening, a laser windowopening, and an etchant opening as described for the vertical antifuseof the second embodiment. The interlayer dielectric within the cavitysidewalls is removed through the etchant opening in the antifuse coverwith a wet etch selective to the interlayer dielectric over the firstmetal segment, the cavity sidewalls, the second metal segment, and theantifuse cover as described in the vertical antifuse of the secondembodiment. An interlayer dielectric (not shown) and an upper contact(282) are also formed as described for the vertical antifuse the secondembodiment. Upper contact (282) can be made from the bottom if thebottom of the cavity is an etch-resistant dielectric such as siliconnitride.

As shown in FIG. 10, a voltage potential (285) is applied between firstmetal segment (221), through lower terminal extension (222), and secondmetal segment (252), through upper contact (282). Voltage potential(285) forms an electrostatic field, and pulsed laser energy is appliedto second metal segment (252) through the laser window opening to softenthe second metal segment (252). The combination of the electrostaticfield and the laser energy causes second metal segment (252) to softenand move into contact with first metal segment (221).

In the horizontal antifuse, second metal segment (252) preferably has alength of less than about 10 microns. The gap between second metalsegment (252) and first metal segment (221) at one sidewall ispreferably about 1 micron, while the gap between second metal segment(252) and first metal segment (221) at the opposite sidewall is at least1 micron greater. For this geometry, the voltage potential and laserenergy recipe of the first embodiment can be used, although it should beunderstood that different geometries can be used by modifying thevoltage potential and laser energy recipes.

Second Embodiment—Multi-State Antifuse

Either the vertical antifuse of the second embodiment or the horizontalantifuse of the second embodiment can be fabricated as a multi-stateantifuse. A third metal segment is formed, whereby the antifuse can beclosed to a first circuit connected to the first metal segment byapplying a voltage potential between the first metal segment and thesecond metal segment, or the antifuse can be closed to a second circuitconnected to the third metal segment by applying a voltage potentialbetween the third metal segment and the second metal segment. Additionalmetal segments beyond the third metal segment can similarly be added toclose additional circuits. The horizontal multi-state antifuse isillustrated in FIGS. 11 and 12.

Referring now to FIGS. 11 and 12, a bottom layer (not shown) for anantifuse cavity (270) is deposited on a semiconductor structure (11). Acavity dielectric (not shown) is formed over the bottom layer. Thedielectric layer is patterned and filled with at least one material withetch selectivity to the dielectric layer, forming cavity sidewalls(291), a first metal segment (221), a second metal segment (252), and athird metal segment (294). The cavity dielectric can be formed inmultiple layers with the cavity sidewalls (291) formed in each layer andthe metal segments (221, 252, 294) can be formed in a middle layer usingdamascene process steps for each layer. The cavity sidewalls (291) andthe metal segments (221, 252, 294) preferably comprise the same metal.Optionally, cavity sidewalls (291) are extended above and below themetal segments (221, 252, 294) using additional metal segments (e.g.,via and wiring metal). In addition, as in all aspects of the secondembodiment, the first cavity sidewalls can comprise dielectric materialwith etch selectivity to the dielectric layer. The metal segments (221,252, 294) are isolated from the bottom layer and the cavity sidewalls bythe cavity dielectric.

An antifuse cover (260) is formed over the multi-state fuse cavity asdescribed for the horizontal antifuse of the second embodiment, forminga cavity (270). The antifuse cover is patterned to form an etchantopening (265). The antifuse cover is formed such that laser energy canbe transmitted through it. The cavity dielectric is removed from thecavity (270) through the etchant opening (265).

An interlayer dielectric layer (280) is formed over the cavity sidewalls(291) and patterned to form wiring (321, 352, 394) to the metal segments(221, 252, 294). The wiring (321, 352, 394) and vias to connect thewiring to the metal segments (221, 252, 294) may be formed using adamascene process as is known in the art. Contacts to the metal segments(221, 252, 294) can be made from the bottom if the bottom of the cavityis an etch-resistant dielectric such as silicon nitride.

The three-state antifuse can be used to close either of two circuits. Acircuit connected to the third metal segment (294) (i.e., terminal 1)can be closed by applying a voltage potential between the second metalsegment (252) and the third metal segment (294) while applying laserpower to the second metal segment (252) through a fuse window (263). Acircuit connected to the first metal segment (291) (i.e., terminal 3)can be closed by applying a voltage potential between the second metalsegment (252) and the first metal segment (291) while applying laserpower to the second metal segment (252) through the fuse window (263).

1. A method of closing an antifuse comprising the steps of: (a)providing an antifuse structure having at least two metal segments witha fuse gap between the metal segments, wherein the fuse gap is an airgap; and (b) simultaneously applying pulsed laser energy to the metalsegments and applying a voltage potential between the metal segments. 2.The method of claim 1 wherein each of the metal segments has an endfacing and defining the fuse gap, at least one of the ends having acorner along the end facing the fuse gap, and wherein the metal segmentssoften upon application of the pulsed laser energy and move to form anelectrical connection upon application of the voltage potential betweenthe metal segments, the corner enhancing the electric field due to thepotential gradient around the corner.
 3. The method of claim 1 whereinthe metal segments comprise at least one of aluminum and copper.
 4. Themethod of claim 1 wherein the density of the applied pulsed laser energyis less than 1 Joule/cm².
 5. The method of claim 1 wherein the appliedvoltage potential is less than 1 volt.
 6. An antifuse structurecomprising: a first metal segment having a first end and being adaptedto be softened upon exposure to pulsed laser energy; a second metalsegment having a second end and being adapted to be softened uponexposure to pulsed laser energy; the first and second ends defining afuse gap between them, wherein the fuse gap is an air gap; and at leastone of the first and second ends having a corner along the end facingthe fuse gap; wherein the first and second metal segments soften uponexposure to pulsed laser energy and move to form an electricalconnection upon application of a voltage potential between the metalsegments, the corner enhancing the electric field due to the potentialgradient around the corner.
 7. The antifuse structure of claim 6 whereinthe metal segments comprise at least one of aluminum and copper.
 8. Theantifuse structure of claim 6 wherein the pulsed laser energy density isless than 1 Joule/cm².
 9. The antifuse structure of claim 6 wherein thevoltage potential is less than 1 volt.
 10. A system for closing anantifuse structure comprising: an antifuse structure including: (a) afirst metal segment having a first end, (b) a second metal segmenthaving a second end, and (c) a fuse gap defined between the first andsecond ends, wherein the fuse gap is an air gap; a pulsed laser sourceapplying a pulsed laser to at least one of the first and second metalsegments to soften the at least one metal segment; and a voltagepotential source applying a voltage between the metal segments whichcauses the at least one softened metal segment to move into contact withthe other metal segment, thereby closing the antifuse structure.
 11. Thesystem of claim 10 wherein at least one of the first and second ends hasa corner along the end facing the fuse gap, the corner enhancing theelectric field due to the potential gradient around the corner.
 12. Thesystem of claim 10 wherein the metal segments comprise at least one ofaluminum and copper.
 13. The system of claim 10 wherein the pulsed laserenergy density is less than 1 Joule/cm².
 14. The system of claim 10wherein the voltage potential is less than 1 volt.